How to create two edges out of an edge?
There can be many ways to model this; if original shape should be preserved, the steps may be as follows.


Select bottom edge which you'd like to make 2 from. Press Alt+D to extend vertices.


Select 2 resulting Ngons, press Ctrl+T to triangulate them, and then Alt+J to get rid of tris and convert them to quads.


Add loopcut to refine model, dissolve resulting and not needed edges.